The detection and mapping of underground cavities is important in mining and geotechnical engineering where load-bearing strength of foundations and potential problems of ground stability and subsidence are of concern. Several geophysical methods have been used in the past for these applications, including gravity and gravity gradient measurements, scalar earth resistivity (DC and low-frequency AC) measurement, audio frequency induction and magnetotelluric measurements, and ground-penetrating radar measurements. All of these methods are potentially capable of detecting an underground cavity if the target is sufficiently large or sufficiently near the surface and each method has been used in certain specialized applications.
To characterize the general performance of these methods, if the cavity target is considered to be a spherical or cylindrical underground void, the detection limits may be expressed in terms of depth-to-diameter ratio at its threshold of detection. When the above-mentioned methods are applied in conventional ways, i.e., operated in a manner corresponding to their usual mineral and geologic exploration applications, test data indicate that these methods can detect cavities at depth limits ranging from about one to three diameters below the surface depending on the technique. The reasons for these detection limitations are somewhat related to the methods themselves, since they are based on different physical principles of operation and, in part, on the masking effects of the geologic structure and/or the surface conditions of the ground medium containing the cavity. This latter effect, generally called geologic noise, affects all of the methods. The superiority of any of the above-discussed detection techniques, based upon its physical method of operation, is less important than its ability to yield a detectable target response in the presence of geologic noise at the survey site.
Deep penetrating resistivity measurements are well known and widely used in borehole geophysics to obtain electrical resistivity logs in which the presence of the borehole fluid or fluid invasions into the drilled formation influence measurements of resistivity in the natural formation. Resistivity measurement techniques designed to provide deep penetration generally employ a multiplicity of electrodes or distributed line electrodes instead of point electrodes to provide a distributed current flow through a relatively large volume of the formation. This arrangement tends to average out the localized effects of geologic noise resulting in a more sensitive resistivity response from regions of the formation located further away from the electrode array. In practice, this process is commonly referred to as a focused resistivity measurement because of the deeper penetration distances achieved and because of the way in which the energized current source electrodes cause the distribution of the measuring current to be confined in the formation. In the common practice of borehole resistivity measurements and applications, the focusing technique causes the current to penetrate to greater radial distances away from the borehole. Focusing also tends to confine the spatial current distribution to a smaller volumetric zone so that the derived resistivity response pertains to a smaller region of the formation than that observed using non-focusing electrode arrays. This concept will become more clearly understood in later discussions in connection with methods used in the present invention to obtain synthetic representations of the current source electrode array.
The application of focusing concepts to surface survey operations introduces several practical problems which are not present in boreholes. First, the survey of underground targets requires the use of long discrete-point electrode arrays in order to achieve the desired depth of penetration. A second concern is that ground contact resistance is significant at surface electrodes, whereas contact resistance is generally negligible in boreholes. In addition to these physical difficulties, effective application of prior focused resistivity methods generally involves significant capital expenses for multiple injection electrode stations and for the increased electrical power required to energize these electrodes simultaneously. The synthetically focused resistivity technique of the present invention, discussed in greater detail below, overcomes these difficulties of the prior art by providing an efficient, cost-effective method for obtaining deep penetrating resistivity measurements which can provide a detectable target response in the presence of geologic noise at the survey site.